1. Field of the Invention
This invention relates to a polymer dispersion containing water, at least one polymer produced by emulsion polymerization, at least one protective colloid, at least one water-soluble acidic metal salt and at least one crosslinking resin based on phenol/formaldehyde condensates, hydroxymethyl-substituted imidazolidinones, such as 1,3-dimethylol-4,5-dihydroxyimidazolidinone (4,5-dihydroxy-N,N′-dimethylolethyleneurea), hydroxymethyl-substituted pyrimidinones or hydroxymethyl-substituted triazinones or self-condensation products thereof or mixed condensates of two or more of the compounds mentioned or a mixture of two or more of the compounds mentioned.
The use of adhesives in the manufacture of industrial goods is increasingly replacing the use of mechanical fastenings, particularly where the connection between two workpieces is exposed to only moderate stresses. The advantages of using adhesives for establishing mechanical bonds lie in particular in the ease of using adhesives, in the possibility of joining materials continuously and, if desired, over entire surfaces and in the prodigious strength of modern adhesives as used, for example, in the furniture industry.
2. Desription of Related Art
Dispersion adhesives are commonly used in the furniture industry, being obtainable for example by emulsion polymerization of ethylenically unsaturated monomers in aqueous phase. Unfortunately, a disadvantage of dispersion adhesives is the fact that they often show poor resistance to moisture or water owing to the presence of emulsifiers and protective colloids in the aqueous dispersion.
GB-A-1,440,337, for example, describes an adhesive which is said to have a long shelf life and to be moisture-resistant. The described adhesive contains a vinyl ester polymer, polyvinyl alcohol as protective colloid, a water-soluble urea/formaldehyde resin with a relatively low degree of condensation and a water-soluble acidic metal salt. However, it is clear from the Examples of the document in question that the corresponding adhesive does not meet the requirements of modern adhesive applications in regard to water resistance.
EP-A 0 191 460 describes polymer emulsions containing polyvinyl alcohol from which coatings with improved resistance to water can be obtained. According to this document, these water-resistant polymer emulsions can be obtained, for example, by polymerizing corresponding ethylenically unsaturated monomers in the presence of polyvinyl alcohol and a redox system. The redox system has to be a nonionic redox system consisting essentially of hydrogen peroxide or an organic peroxy compound as oxidizing agent and ascorbic acid or erythorbic acid as reducing agent. However, even after crosslinking with glutaraldehyde, the described coatings do meet the requirements of DIN EN 204 D3 or D4.
EP-B 0 686 682 relates to a polyvinyl ester-based dispersion adhesive of which films show high resistance to water. An aqueous polyvinylester dispersion is used for this purpose which, besides polymeric protective colloids, contains water-soluble compounds “complexable” with the polymeric protective colloids and derivatives of polyaldehydes from which aldehyde groups can be released under control in acidic media. However, it is not proposed in the cited document to use a crosslinking resin based on phenol/formaldehyde condensates, hydroxymethyl-substituted imidazolidinones, such as 1,3-dimethylol-4,5-dihydroxyimidazolidinone (4,5-dihydroxy-N,N′-dimethylolethyleneurea) or dihydroxymethyl ethyleneurea, dihydroxymethyl propyleneurea or dihydroxymethylurone, hydroxymethyl-substituted pyrimidinones or hydroxymethyl-substituted triazinones or thio derivatives thereof, self-condensation products thereof or mixed condensates of two or more of the compounds mentioned or a mixture of two or more of the compounds mentioned as the crosslinking resin.
A disadvantage of the prior art as cited above is in particular the fact that the described adhesives either do not meet water resistance standards (DIN EN 204 D3 or D4/5) or require low molecular weight polyaldehydes either in free from or, for example, as a bisulfite adduct as crosslinking agent. Problems arise out of the fact that the polyaldehydes mentioned or their adducts generally have a strong odor which can be troublesome or can even pose a health risk.
Accordingly, the problem addressed by the present invention was to provide a polymer dispersion which, in the form of the films produced from and, in particular, bonds established with the polymer dispersion, would show improved resistance to moisture or water, especially hot water, and which would not require the presence of free aldehydes or bisulfite adducts thereof.
This problem has been solved by a polymer dispersion containing water, an emulsion polymer, a protective colloid, a water-soluble acidic metal salt and a crosslinking resin.